There are a lot of hobbyists (with a lot of electrical savvy) who are trying to copy the hacks made by Calcars and the rest.
When they sort out the details, they are going to make the plans available. The early Calcars Prius+ was lead-acid, so you could DIY a much cheaper PHEV-Prius conversion using, say 3 kWh of lead-acid? Would give you 10 miles EV each day, but cost only $1,000 for the batteries, some extra for the charger and controller etc.
You can read about the lead-acid version here:
http://www.calcars.org/priusplus.html (Detailed tech messages and info at yahoo group priusplus)
Alternatively, you can follow the DIYers here:
http://www.eaa-phev.org/wiki/Main_Page
Very early days for PHEV right now. Don't worry though, it's going to get real cheap, real soon.